Update plugins to their latest release versions. Resolves #17

mvn versions:display-plugin-updates output:

The following plugin updates are available:
  maven-acr-plugin ......................................... 1.0 -> 1.1
  maven-assembly-plugin .................................. 2.4 -> 2.4.1
  maven-changelog-plugin ................................... 2.2 -> 2.3
  maven-deploy-plugin .................................. 2.8.1 -> 2.8.2
  maven-ear-plugin ....................................... 2.9 -> 2.9.1
  maven-ejb-plugin ......................................... 2.3 -> 2.4
  maven-install-plugin ................................. 2.5.1 -> 2.5.2
  maven-invoker-plugin ..................................... 1.8 -> 1.9
  maven-jar-plugin ......................................... 2.4 -> 2.5
  maven-pmd-plugin ......................................... 3.1 -> 3.2
  maven-rar-plugin ......................................... 2.3 -> 2.4
  maven-scm-plugin ....................................... 1.9 -> 1.9.1
  maven-site-plugin ........................................ 3.3 -> 3.4
  maven-source-plugin .................................... 2.2.1 -> 2.3
  org.codehaus.mojo:build-helper-maven-plugin ............ 1.8 -> 1.9.1
  org.codehaus.mojo:findbugs-maven-plugin .............. 2.5.4 -> 3.0.0