suppress build warnings

plugin versions need to be explicitly specified to suppress following warning messages:

[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.unigram:redpen-core:jar:0.6
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 44, column 17
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.unigram:redpen-app:jar:0.6
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 23, column 17
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.unigram:redpen-server:war:0.6
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 18, column 15
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-surefire-plugin is missing. @ line 93, column 15
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-war-plugin is missing. @ line 35, column 15
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]