gem 'rb-gsl'

[ci skip]
Install GSL before running tests on Travis
Refactor tests for AO::Gaussian::PrimitiveProduct
Fix Gemfile to require gsl
Refactor tests for PrimitiveProduct#hermitian_coeff_decomposed
Fix wrong orders
Refactor test codes for PrimitiveProduct
Fix the test using Hermite polynomials
Test hermitian_coeff_decomposed for each direction
Merge branch 'feature/hermite_test' into develop