Changing the way of searching for Eclipse JDT jar.

This change should simplify the way plugin looks for JDT jar. It will also fix the situation when JDT jar is on different location then "<installation_root>/plugins" (that happens if you use tycho-surefire-plugin).

The methods used were introduced in version 3.4 and sooner, so there should not be a problem with compatibility. There is also no need for searching through files multiple times as the supplied method returns only the bundle with highest version available.