Use maven-compiler-plugin 3.1 in compiler IT tests to fix platform-dependent issues

See https://plus.google.com/u/0/116467112276686980985/posts/EZMtR3D6azL

With maven-compiler-plugin 2.3.2 (the default for Maven 3.0.4 and 3.0.5), on my Linux box one IT fails because the last compiler error only has its first line printed in the Maven output, so the check in `verify.bsh` fails. On a Windows box (see original report on G+ linked above), all compiler errors have only their first line printed, so an additional test fails (but worked for me because the `verify.bsh` only looks at lines from the first few errors, not the last one).

Bumping to 3.x (tested 3.0 and 3.1) fixes the issue on my Linux box.

```
$ mvn -version
Apache Maven 3.0.5 (r01de14724cdef164cd33c7c8c2fe155faf9602da; 2013-02-19 14:51:28+0100)
Maven home: /home/tbr/bin/apache-maven-3.0.5
Java version: 1.7.0_21, vendor: Oracle Corporation
Java home: /usr/lib/jvm/java-7-openjdk-amd64/jre
Default locale: fr_FR, platform encoding: UTF-8
OS name: "linux", version: "3.8.0-19-generic", arch: "amd64", family: "unix"
```