Automake 1.11 is the first version to support AM_PROG_VALAC

According to Autmake NEWS file, the first version of automake to have
vala support is 1.11.  This requires to bump REQUIRED_AUTOMAKE_VERSION
from 1.10 to 1.11.
Merge pull request #18 from yashi/master

Automake 1.11 is the first version to support AM_PROG_VALAC