Don't depend on bcprov-jdk15on and bcprov-ext-jdk15on

[TobiX]

Bouncycastle provides the artifacts bcprov-jdk15on and bcprov-ext-jdk15on, where the ext is a superset of the non-ext artifact, so depending on both is pointless. As the only difference currently seems to be the NTRU algorithm (see http://bouncycastle.org/latest_releases.html), I would suggest to just depend on the non-ext artifact and users who need that algorithm can do the song-and-dance to switch out the dependency.