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.
Run the TravisCI builds on MacOSX

Unfortunately TravisCI does not allow to test the same branch on both
Linux and MacOSX...