Cantera 2.5.1 release

Cantera 2.5.0 release

Release candidate for Cantera 2.5.0

[SCons/Doc] Fix installation of Doxygen docs

Fixes regression introduced in b84d3e3, where the organization of files in the built
documentation changed.

Fixes Cantera#553

[Doc] Update versions of Ubuntu for which Cantera is packaged

We can no longer build on stock 14.04 due to the requirement for Cython >= 0.23.

[CI] Return exit code from 'scons test' to make Appveyor status correct

Resolves Cantera#382

[Doc] Update branch in git checkout instructions to 2.2

Bump version to 2.2.0

Bump version to 2.2.0b1