travis.yml: Fix the ccache lines

travis.yml: Fix the ccache lines

The "command -v ccache && ccache ..." likely were supposed to test
the availability of ccache before running the program. But this
shell construct causes Travis to abort if ccache is not available.
Use an if-statement instead to fix this problem.

Signed-off-by: Thomas Huth <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20191009170701[email protected]>
Signed-off-by: Alex Bennée <[email protected]>