Travis: move faster builds to baseline

- use py27-pexpect,py27-trial,py27-numpy and py37-xdist in baseline,
  using pexpect there catches errors with pdb tests early, and
  py37-xdist is much faster than py37.
- move py34 and py36 out of baseline.

.travis.yml

@@ -13,14 +13,13 @@ install:
   - pip install --upgrade --pre tox
 env:
   matrix:
+    - TOXENV=py27
     # Specialized factors for py27.
-    - TOXENV=py27-pexpect,py27-trial,py27-numpy
     - TOXENV=py27-nobyte
     - TOXENV=py27-xdist
     - TOXENV=py27-pluggymaster PYTEST_NO_COVERAGE=1
     # Specialized factors for py37.
     - TOXENV=py37-pexpect,py37-trial,py37-numpy
-    - TOXENV=py37-xdist
     - TOXENV=py37-pluggymaster PYTEST_NO_COVERAGE=1
     - TOXENV=py37-freeze PYTEST_NO_COVERAGE=1
 
@@ -30,8 +29,12 @@ jobs:
     - env: TOXENV=pypy PYTEST_NO_COVERAGE=1
       python: 'pypy-5.4'
       dist: trusty
+    - env: TOXENV=py34
+      python: '3.4'
     - env: TOXENV=py35
       python: '3.5'
+    - env: TOXENV=py36
+      python: '3.6'
     - env: TOXENV=py37
     - &test-macos
       language: generic
@@ -50,11 +53,8 @@ jobs:
         - brew link python
 
     - stage: baseline
-      env: TOXENV=py27
-    - env: TOXENV=py34
-      python: '3.4'
-    - env: TOXENV=py36
-      python: '3.6'
+      env: TOXENV=py27-pexpect,py27-trial,py27-numpy
+    - env: TOXENV=py37-xdist
     - env: TOXENV=linting,docs,doctesting
       python: '3.7'