Pin r10k to 2.6.4

Change-Id: I4875b8bc8b2333046fc3a08b4669774fd26c89cb
(cherry picked from commit 3d5f3ba)

Author: tobias-urdin

install_modules.sh

@@ -28,7 +28,7 @@ gem install fast_gettext -v '< 1.2.0' --verbose
 # puppet_forge 2.2.7 has a dependency on semantic_puppet ~> 1.0
 # which is not compatible with dependency of latest r10k on semantic_puppet ~> 0.1.0
 gem install puppet_forge -v '= 2.2.6' --verbose
-gem install r10k --no-ri --no-rdoc --verbose
+gem install r10k -v 2.6.4 --no-ri --no-rdoc --verbose
 
 # make sure there is no puppet module pre-installed
 rm -rf "${PUPPETFILE_DIR:?}/"*