travis: switch to ubuntu trusty
travis: switch to ubuntu trusty

ubuntu trusty has updated doxygen 1.8.4 which is the same
stable version 1.8 which is described in DEPENDANCIES file.
Now doxygen do not generate any warnings. Compiler version
stays the same 4.8 for gcc.

Signed-off-by: Maxim Uvarov <db4d16e02ae2d7493db430203537da8b2e34f290@linaro.org>
Reviewed-by: Mike Holmes <ed44c09f4d7f698f5510adc894a872c73e08c8bd@linaro.org>

dkrot/odp,dkrot/odp,dkrot/odp,dkrot/odp