Expand Travis build matrix

[David Dickinson]

The Travis CI service has recently been introduced for use with next and future releases. This can be seen as a more open alternative for the current Jenkins CI service provided by Anvil at STFC. Before we can fully remove Jenkins we should ensure Travis provides the same build cases. Specifically we should have

1. [x] Default build with default gfortran compiler
2. [X] Default build with newest easily available gfortran compiler
3. [X] Default build with intel compiler

It would also be of interest to have a wider range of compilers, a no dependency build and a debug build.

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[David Dickinson]

PR #331 should fix this issue

[David Dickinson]

No longer intending to use Travis due to change in open source policy. Expanded gitlab builds in PR #385 instead.