Compatibility with Maven 3.1.x

Launching build with Maven 3.1.x finish with an error with Aether:
```
Exception in thread "main" java.lang.NoSuchMethodError: org.apache.maven.execution.MavenSession.getRepositorySession()Lorg/sonatype/aether/RepositorySystemSession;
        at org.eclipse.tycho.core.maven.utils.PluginRealmHelper.execute(PluginRealmHelper.java:86)
...
```
This error is related to https://cwiki.apache.org/confluence/display/MAVEN/AetherClassNotFound. Solution seems easy: change tycho version from 0.18.0 to 0.18.1. With this change plugin is successfully built in Maven 3.1.x (I have no test other versions of maven).