travis: use released version Groonga's

Mroonga doesn't work with Groonga
d57d1a5602044f24c7631ce58d9bec169586a58a or later. Because wrapper
mode uses forward index style. Umm...

tools/travis/install.sh

@@ -21,8 +21,8 @@ set -e
 
 mariadb_download_base=http://mirror.jmu.edu/pub/mariadb
 
-export GROONGA_MASTER=yes
-export GROONGA_NORMALIZER_MYSQL_MASTER=yes
+# export GROONGA_MASTER=yes
+# export GROONGA_NORMALIZER_MYSQL_MASTER=yes
 
 curl https://raw.github.com/groonga/groonga/master/data/travis/setup.sh | sh
 curl https://raw.github.com/groonga/groonga-normalizer-mysql/master/data/travis/setup.sh | sh