Restrict google-protobuf dependency to version < 3.15

Gitlab does not work with google-protobuff 3.15.x.
Using the new version causes segfaults:
/usr/local/lib/libruby27.so.27(rb_profile_frame_full_label+0x1665)
   [0x80059c535]
/usr/local/lib/libruby27.so.27(0x8003fa3fb) [0x8003fa3fb]
/usr/local/lib/libruby27.so.27(0x8005094d2) [0x8005094d2]
/lib/libthr.so.3(_pthread_sigmask+0x53e) [0x800649b7e]
/lib/libthr.so.3(0x80064913f) [0x80064913f]
   [0x7ffffffff193]
/usr/local/lib/ruby/gems/2.7/gems/google-protobuf-3.15.0/lib/google/
	   protobuf_c.so(Defs_register+0xc99) [0x811958699]
/usr/local/lib/ruby/gems/2.7/gems/google-protobuf-3.15.0/lib/google/
   protobuf_c.so(0x81195c675) [0x81195c675]

For more details see this bug report:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254010