Extend support for Travis-CI and use caching with ccache

[robertodr]

The .travis.yml is based on work by @daniel-j-h and performs tests
on Linux and OS X for a number of Clang and GCC compiler versions.
The full list of testing environments is in the README.md file.

A number of criticalities in the build processes showed up. Some I think are artifacts on Travis, others are indeed bugs.

Clang builds on Linux fail with:

error: unknown type name '__extern_always_inline'

the #include <cmath> triggers this. I suspect incosistencies between the compiler version and the Standard C++ Library.

GCC 4.7 in release mode on Linux fails with:

Start 1: cpp_test
1: Test command: /home/travis/build/robertodr/numgrid/build/bin/cpp_test
1: Test timeout computed to be: 1500
test 2
    Start 2: fortran_test
2: Test command: /home/travis/build/robertodr/numgrid/build/bin/fortran_test
2: Test timeout computed to be: 1500
1: [==========] Running 2 tests from 1 test case.
1: [----------] Global test environment set-up.
1: [----------] 2 tests from numgrid
1: [ RUN      ] numgrid.h2o
1: /home/travis/build/robertodr/numgrid/test/test.cpp:142: Failure
1: Value of: fabs(error) < 1.0e-6
1:   Actual: false
1: Expected: true
1: [  FAILED  ] numgrid.h2o (96 ms)
1: [ RUN      ] numgrid.o_in_h2o
2: STOP 1
1/2 Test #2: fortran_test .....................***Failed    0.14 sec
1: [       OK ] numgrid.o_in_h2o (43 ms)
1: [----------] 2 tests from numgrid (139 ms total)
1: 
1: [----------] Global test environment tear-down
1: [==========] 2 tests from 1 test case ran. (139 ms total)
1: [  PASSED  ] 1 test.
1: [  FAILED  ] 1 test, listed below:
1: [  FAILED  ] numgrid.h2o
1: 
1:  1 FAILED TEST
2/2 Test #1: cpp_test .........................***Failed    0.14 sec

This might be a genuine bug.

Builds on Mac fail because HomeBrew fails in installing Valgrind.

[bast]

Thanks a lot! I need to look at it in bit more detail also because of coverage. Very much appreciated.

[bast]

Thanks! I put this temporarily on a branch roberto/travis. I need to fix the coverage part. The changes are very much appreciated and will soon make it to master. Closing the PR.