MRI only gem packages

[ci skip]
Fix a spec
Skeleton for ruphy_libint2 extension
Find libint2 library
Use extconf.h
Define module RuPHY::Libint2
Define RuPHY::Libint2::compiled?
Call libint2_static_init/cleanup
Add autotest mapping for extensions
Define RuPHY::Libint2::Evaluator

[ci skip]
Initialize eri evaluator

[ci skip]
Use ruby_xfree to release evaluator buffer

[ci skip]
Install libint2 from ppa before CI build
Search for libint2.h
Add *.cc to libint2 extention source
Spec for Libint2::Evaluator#max_angular_momentum
Fix bug around Libint2::Evaluator@max_angular_momentum
ruby 1.9.2 workaround
Compile the main source file as C++
Use pkg-config