upgrade tycho as the 0.18.1 is not compatible with maven 3.1.X

when running against : maven 3.1.0
you get the following error:
initializationError(org.eclipse.tycho.core.test.DefaultBundleReaderTest): org/sonatype/aether/RepositorySystemSession

bug link: https://bugs.eclipse.org/bugs/show_bug.cgi?id=406056
Merge pull request #11 from wordstobytes/master

upgrade tycho as the 0.18.0 is not compatible with maven 3.1.X