fix conflicting "using std" declaration with "using boost::thread"

[matlo607]

Summary

I experienced issues while building cucumber-cpp with the environment below.

Environment:

Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.3 LTS
Release:	16.04
Codename:	xenial

gcc (Ubuntu 5.4.1-2ubuntu1~16.04) 5.4.1 20160904
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Boost 1.66.0

Errors:

/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:65:29: error: template argument 1 is invalid
     boost::scoped_ptr<thread> serverThread;
                             ^
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp: In member function ‘virtual void SocketServerTest::SetUp()’:
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:69:22: error: request for member ‘reset’ in ‘((SocketServerTest*)this)->SocketServerTest::serverThread’, which is of non-class type ‘int’
         serverThread.reset(new thread(&SocketServer::acceptOnce, server));
                      ^
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:69:32: error: expected type-specifier before ‘thread’
         serverThread.reset(new thread(&SocketServer::acceptOnce, server));
                                ^
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:68:23: error: unused variable ‘server’ [-Werror=unused-variable]
         SocketServer* server = createListeningServer();
                       ^
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp: In member function ‘virtual void SocketServerTest::TearDown()’:
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:74:25: error: base operand of ‘->’ is not a pointer
             serverThread->timed_join(THREAD_TEST_TIMEOUT);
                         ^
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:75:26: error: request for member ‘reset’ in ‘((SocketServerTest*)this)->SocketServerTest::serverThread’, which is of non-class type ‘int’
             serverThread.reset();
                          ^

using std; creates a conflict with using boost::thread; because std has also a class thread that must have been previously included by boost/thread.hpp

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• I've added tests for my code.
• I have verified whether my change requires changes to the documentation
• My change either requires no documentation change or I've updated the documentation accordingly.
• My branch has been rebased to master, keeping only relevant commits.

[matlo607]

@paoloambrosio, @muggenhor or @konserw : could you please review the pull request ?
Please ignore the commit 08049bd, konserw told me it's wrong. I will wait for #184.

[coveralls]

Coverage remained the same at 63.329% when pulling f92bdac on matlo607:fix-using-namespace into 70fa12c on cucumber:master.

[konserw]

Merged as part of #189