.travis.yml: specify custom build matrix

This build matrix explicitly tests dash, bash, and zsh (on Python 3.5),
allows nightly, pypy, and pypy3 to fail, and adds Python 2.4 and 2.5
builds.

PyPy sometimes fails in random spots due to a bug in its subprocess
library (where Popen.communicate() can raise IOError/EPIPE when closing
stdin). Therefore, these builds are allowed to fail (and they also don't
block the overall build from being marked as finished).

The Python 2.4/2.5 builds do not measure test coverage or run dist.t,
pep8.t, or pyflakes.t. Those require installing things using pip which
is annoying and maybe impossible on Travis.

.travis.yml

@@ -1,23 +1,67 @@
 language: python
 
-python:
-    - "2.6"
-    - "2.7"
-    - "3.2"
-    - "3.3"
-    - "3.4"
-    - "3.5"
-    - nightly
-    - pypy
-    - pypy3
+matrix:
+  allow_failures:
+    - python: nightly
+      env: TESTOPTS=--shell=dash
+    - python: pypy
+      env: TESTOPTS=--shell=dash
+    - python: pypy3
+      env: TESTOPTS=--shell=dash
+  include:
+    - python: "3.5"
+      env: TESTOPTS=--shell=dash
+    - python: "3.5"
+      env: TESTOPTS=--shell=bash
+    - python: "3.5"
+      env: TESTOPTS=--shell=zsh
+      addons:
+        apt:
+          packages:
+            - zsh
+    - python: "3.4"
+      env: TESTOPTS=--shell=dash
+    - python: "3.3"
+      env: TESTOPTS=--shell=dash
+    - python: "3.2"
+      env: TESTOPTS=--shell=dash
+    - python: "2.7"
+      env: TESTOPTS=--shell=dash
+    - python: "2.6"
+      env: TESTOPTS=--shell=dash
+    - env: PYTHON=2.5 TESTOPTS=--shell=dash
+      addons:
+        apt:
+          sources:
+            - deadsnakes
+          packages:
+            - python2.5
+    - env: PYTHON=2.4 TESTOPTS=--shell=dash
+      addons:
+        apt:
+          sources:
+            - deadsnakes
+          packages:
+            - python2.4
+    - python: nightly
+      env: TESTOPTS=--shell=dash
+    - python: pypy
+      env: TESTOPTS=--shell=dash
+    - python: pypy3
+      env: TESTOPTS=--shell=dash
+  fast_finish: true
 
 install: |
-    if [ "$TRAVIS_PYTHON_VERSION" = "3.2" ]
-    then
-        pip install coverage==3.7.1
-    else
-        pip install coverage
-    fi
+  if [ -z "$PYTHON" ]
+  then
+    [ "$TRAVIS_PYTHON_VERSION" = "3.2" ] && pip install coverage==3.7.1
     pip install -r requirements.txt
+  fi
 
-script: make test
+script: |
+  if [ -z "$PYTHON" ]
+  then
+    make test TESTOPTS="$TESTOPTS"
+  else
+    make quicktest PYTHON="python$PYTHON"
+  fi