Update submodule netcipher, setup tracking branch 'master'

Make use of git >= 1.8.2 feature to track submodule brach and allow easy updates by

    git submodule update --remote

More info: http://www.vogella.com/tutorials/Git/article.html#submodules

.gitmodules

@@ -1,3 +1,4 @@
 [submodule "external/netcipher"]
 	path = external/netcipher
 	url = https://github.com/guardianproject/NetCipher.git
+	branch = master