Change from gmaven to groovy-eclipse-compiler.

GMaven seems to be discontinued (see http://groovy.github.io/gmaven/) in favour of groovy-eclipse-compiler, which is better supported by (recent versions of) Eclipse and m2e anyway.

It requires the update to maven-compiler-plugin 3.1; whether this or the use of groovy-eclipse-compiler I'm not sure but maven now prints a lot of compiler warnings which weren't there before (even though you already had showWarnings=true).

For simplicity I've set the groovy-eclipse-compiler to be used by default whenever maven-compiler-plugin runs (i.e. compile and testCompile), and this works fine in all cases except where there is no src/xyz/java in which case it doesn't spot src/xyz/groovy; there are a few ways round this (described on http://groovy.codehaus.org/Groovy-Eclipse+compiler+plugin+for+Maven) and I've picked one of them and just added it to thucydides-easyb-plugin for the moment.  Could move this to the top-level POM if needed.