Run Travis tests with 2 MPI ranks

[RonRahaman]

Traditionally, Nek's parallel test cases have been run with 4 MPI ranks. However, each Travis virtual environment only provides a maximum of 2 cores (reference). This likely contributes to the considerable slowdown of Nek's parallel test cases in Travis.

As @stgeke suggested, we could probably improve turnaround time of the Travis tests by running the Nek tests with only 2 MPI ranks.

To do so, we must:

• Allow the user to set MPI ranks in NekExamples/NekTests.py (see Allow user to set MPI ranks in test suite NekExamples#15)
• Update .travis.yml to use 2 MPI ranks