Test YCM on multiple CMake versions using Travis CI

[diegoferigo]

As discussed with @drdanz, I'm extending Travis CI test capabilities. I have some questions before proceeding:

• Is testing on linux enough or osx is also recommended?
• If yes, is it useful testing with Xcode generator?
• Is it useful testing either Debug and Release build types?
• Is it useful testing either with gcc and clang?

[traversaro]

My take:

• macOs testing is definitely recommended (YES)
• Xcode is a big pain point, so testing the Xcode generator is highly beneficial (YES)
• Not really important, as no C/C++ code is compiled by YCM, so there should be no difference in the two builds (MEH/NO),
• Not really important, as (I far as I know) the compiler is never called by YCM (MEH/NO).

[diegoferigo]

macOS was already tested before, but only with the CMake version included in default Travis' osx virtualization environment. #115 could be used as a scaffold for implementing the test of multiple CMake versions also in osx.

[drdanz]

I agree with @traversaro

• Is testing on linux enough or osx is also recommended?

osx is recommended

• If yes, is it useful testing with Xcode generator?

I'd say that we should test at least "Unix Makefiles" and "Ninja" on linux, "Xcode" on macOS

• Is it useful testing either Debug and Release build types?

No, it's actually useless, you can just use Release

• Is it useful testing either with gcc and clang?

No, it's more important to test different generators...

macOS was already tested before, but only with the CMake version included in default Travis' osx virtualization environment. #115 could be used as a scaffold for implementing the test of multiple CMake versions also in osx.

On macOS we don't care about multiple CMake versions, as long as the latest version works.

We are supporting older cmake versions for linux only (see http://wiki.icub.org/wiki/YARP_Supported_Distributions)