Travis-CI: bump aexpect requirement to 1.4.0

Because that version contains Python 3 syntax fixes.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

requirements-travis.txt

@@ -11,7 +11,7 @@ snakefood==1.4; python_version <= '2.7'
 networkx==1.9.1; python_version <= '2.7'
 pygraphviz==1.3rc2; python_version <= '2.7'
 mock==1.2.0; python_version <= '2.7'
-aexpect==1.0.0
+aexpect==1.4.0
 psutil==3.1.1
 # six is a stevedore depedency, but we also use it directly
 six==1.9.0