Pin the pants binary to osx and linux platforms.

Also update to coverage 3.7.1 which has a good set of
osx and linux eggs deployed at:
  https://github.com/twitter/commons/blob/binaries/pants/third_party/python/dist/

(sapling split of a1dc1716e7b323e8083f80dbe1ceb72ec57e9883)

3rdparty/python/BUILD

@@ -31,7 +31,7 @@ def make_dep(name, version, dependency_name=None):
 make_dep('ansicolors', '1.0.2')
 make_dep('antlr-3.1.3', '3.1.3', 'antlr_python_runtime')
 make_dep('bottle', '0.11.6')
-make_dep('coverage', '3.6b1')
+make_dep('coverage', '3.7.1')
 make_dep('elementtree', '1.2.7_20070827_preview')
 make_dep('futures', '2.1.2')
 make_dep('git-python', '0.3.2.RC1', 'GitPython')

src/python/twitter/pants/bin/BUILD

@@ -34,6 +34,11 @@ python_library(
 python_binary(
   name = 'pants',
   entry_point = 'twitter.pants.bin.pants_exe:main',
+  platforms=[
+    'current',
+    'linux-x86_64',
+    'macosx-10.4-x86_64',
+  ],
   dependencies = [
     pants(':pants_exe'),
     pants('src/python/twitter/pants/commands:pants_old'),