MySQL 5.7 is GA now, remove the -DMR suffix from the apt repo

Author: sodabrew

.travis_mysql57.sh

@@ -6,7 +6,7 @@ service mysql stop
 apt-get purge '^mysql*' 'libmysql*'
 apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:80 0x8C718D3B5072E1F5
 
-add-apt-repository 'deb http://repo.mysql.com/apt/ubuntu/ precise mysql-5.7-dmr'
+add-apt-repository 'deb http://repo.mysql.com/apt/ubuntu/ precise mysql-5.7'
 
 apt-get update
 apt-get -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confnew -y install mysql-server libmysqlclient-dev