cantera: Update to version 2.1.1

Replace the old formula for the 1.8 snapshot with the current release. The
new version has a different build system, so the install method needed to
be rewritten.

Add support for building Matlab toolbox (--with-matlab=...) and Python
3 module (--with-python3).

Add support for building unstable branch (--HEAD) and maintenance
branch (--devel).