Drop Python 3.4 support

PyPI stats indicates that 0.00% of downloads are for Python 3.4:
https://www.pypistats.org/packages/mypy-extensions

Other supported Python versions have non-trivial downloads, so let's
continue to support them for now.
diff --git a/setup.py b/setup.py
index c0b5d9e..2254d21 100644
--- a/setup.py
+++ b/setup.py
@@ -20,7 +20,6 @@
     'Programming Language :: Python :: 2',
     'Programming Language :: Python :: 2.7',
     'Programming Language :: Python :: 3',
-    'Programming Language :: Python :: 3.4',
     'Programming Language :: Python :: 3.5',
     'Programming Language :: Python :: 3.6',
     'Programming Language :: Python :: 3.7',
diff --git a/tox.ini b/tox.ini
index fe02312..564b071 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.9.1
 skip_missing_interpreters = true
-envlist = py27, py34, py35, py36, py37
+envlist = py27, py35, py36, py37
 
 [testenv]
 description = run the test driver with {basepython}