Merge remote-tracking branch 'danaj/master'
[travis] Newer Math::GMPz's require GMP 4.2.x or higher and install build time deps at the proper time
Add some failing tests for is_strong_pseudoprime pointed out by @danaj
Make is_strong_pseudoprime tests pass and change +1/-1 to be considered strong pseudoprimes, fixes #12
Merge branch 'pseudoprime_bases'
[travis] Try using GMP 5.0