travis: remove python 2.7 and 3.4, add 3.7

Signed-off-by: Karol Gugala <kgugala@antmicro.com>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 0a1062c..d5791ae 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,14 +2,12 @@
 
 matrix:
   include:
-    - python: 2.7
-      env: TOXENV=py27
-    - python: 3.4
-      env: TOXENV=py34
     - python: 3.5
       env: TOXENV=py35
     - python: 3.6
       env: TOXENV=py36
+    - python: 3.7
+      env: TOXENV=py37
 
 install: pip install tox