Prune Travis CI build menu

[robertodr]

In order to speed up check times on PRs, a significant number of builds have been removed from .travis.yml.
The rationale is that we want to test the oldest supported compilers on Linux and fairly recent compilers on Mac OS X, where it is easier to get update versions of the build toolchain.

Description

As documented in the README.md the following have been left:

• Ubuntu 12.04 LTS 64-bit with CMake 3.3.2 and Boost 1.55.0 this is the
  environment offered by Travis CI pulling in various
  PPA. Python and Python packages are installed and managed via Conda within
  an environment defined in the .pcmsolver-travis.yml file. The following
  compilers are used:

  1. GCC 4.6, Python 2.7 This build generates both the shared and static
     libraries, linking executables to the former. The build is run with and
     without coverage analysis.
  2. Clang 3.5, GFortran 4.6, Python 3.5 This build generates only the static
     library.

• Mac OS X 10.11 with CMake 3.6.2 and Boost 1.61.0
  this is the environment offered by Travis CI
  with their Xcode 7.3.1 image.
  The following compilers are used:

  1. Apple LLVM 7.3.0, GFortran 5.4.0, Python 2.7 This build generates only
     the static library.
  2. GCC 6.2.0, Python 3.5 This build generates both the shared and static
     libraries, linking executables to the former.

Todos

• Developer Interest

• Leaner Travis CI build menu.
• Code is now tested serially. This should hopefully the annoying, nondeterministic failures on OS X.
• Build reports not sent to dashboard anymore.

Status

• Ready to go

[ilfreddy]

@robertodr do you need somebody else's approval or can I merge?

[robertodr]

This is trivial enough to be accepted with just one review 😺 Thanks!