Use mktemp instead of $RANDOM to generate unique names

Using $RANDOM is not safe because collisions can occur. An example of
such a failure is available here:
https://s3.amazonaws.com/archive.travis-ci.org/jobs/23572639/log.txt

Using mktemp avoids this issue and removes the dependency on bash.
Working TravisCI configuration

Add a minimal configuration file for TravisCI.

This builds and runs the tests for:
 - the runtime
 - the compiler
 - the core library (for profiles 2.0, 4.0 and 4.5)
 - the class library (for profiles 3.5 and XBuild 12)
on Linux, using both gcc and clang.