.travis.yml: add travis dependencies for plainbox

This patch ensures that we install/upgrade lmxl and xlsxwriter that are
needed by plainbox.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
.travis.yml: enable testing on python 3.4

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/travis/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"