[install] Use Scala 2.11, not Scala 2.7

When I used Scala 2.7, I got a compiler error "not found: value math."
As far as I can tell, this is because Scala 2.7 doesn't include a
scala.math package (http://www.scala-lang.org/api/2.7.7/).  I tried
Scala 2.11.7 instead, and that worked fine.

INSTALL

@@ -2,7 +2,7 @@ INSTALLATION INSTRUCTIONS
 
 INSTALLING ON POSIX COMPATIBLE SYSTEMS
 
-* Install the dependencies: Scala 2.7, Sun Java JDK 7, libmysql-java, mysql
+* Install the dependencies: Scala 2.11, Sun Java JDK 7, libmysql-java, mysql
 
 * Edit the file bin/exports.sh (and optionally
   etherpad/bin/etherpad.default) and change the paths to