Automake 1.11 is the first version to support AM_PROG_VALAC

According to the [Autmake NEWS][1] file, the first version of Automake to have Vala support is 1.11.  This requires to bump the REQUIRED_AUTOMAKE_VERSION from 1.10 to 1.11.

[1]: http://git.savannah.gnu.org/cgit/automake.git/tree/NEWS?id=7fa2890c4ec91ae20589eee67eef0be9bcbe159b

Here is a log running the autogen on a system with Automake 1.10 only.

```
Running aclocal-1.10...
main::scan_file() called too early to check prototype at /usr/bin/aclocal-1.10 line 616.
configure.ac:72: warning: macro `AM_PROG_VALAC' not found in library
Running autoconf...
configure.ac:72: error: possibly undefined macro: AM_PROG_VALAC
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
```