DEV: Don't invoke hardwired Python

Instead use /urs/bin/env to detect the Python interpreter. This way the
scripts work better with the possible virtual environment.

tests/test_algorithm_gen.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2013 Quantopian, Inc.
 #

zipline/examples/buyapple.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2012 Quantopian, Inc.
 #

zipline/examples/dual_ema_talib.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2013 Quantopian, Inc.
 #

zipline/examples/dual_moving_average.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2013 Quantopian, Inc.
 #

zipline/examples/pairtrade.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2013 Quantopian, Inc.
 #