Updates flake8 to latest.

The latest flake8 release in now 1.5, which pulls in pep8: 1.3.4a0

The upgrade pep8 has changes to what it picks up as lint.
Making code base compatible, so that new devs can install pep8
from PyPI and not have friction over the version difference.

Currently using these ignores in the config file:

```
[pep8]
ignore = E124,E125,E126
```

Ignoring these since they are difficult to squash while maintaining
an 80 char line length, and appear spurious.
Should address later.

Updates Travis config, README, and pip requirements to reflect change.

zipline/algorithm.py

@@ -134,8 +134,8 @@ class TradingAlgorithm(object):
         """
         if isinstance(source, (list, tuple)):
             assert start is not None and end is not None, \
-            """When providing a list of sources, \
-start and end date have to be specified."""
+                """When providing a list of sources, \
+                start and end date have to be specified."""
         elif isinstance(source, pd.DataFrame):
             assert isinstance(source.index, pd.tseries.index.DatetimeIndex)
             # if DataFrame provided, wrap in DataFrameSource
@@ -234,14 +234,14 @@ start and end date have to be specified."""
 
     def set_slippage(self, slippage):
         assert isinstance(slippage, (VolumeShareSlippage, FixedSlippage)), \
-                MESSAGES.ERRORS.UNSUPPORTED_SLIPPAGE_MODEL
+            MESSAGES.ERRORS.UNSUPPORTED_SLIPPAGE_MODEL
         if self.initialized:
             raise Exception(MESSAGES.ERRORS.OVERRIDE_SLIPPAGE_POST_INIT)
         self.slippage = slippage
 
     def set_commission(self, commission):
         assert isinstance(commission, (PerShare, PerTrade)), \
-                MESSAGES.ERRORS.UNSUPPORTED_COMMISSION_MODEL
+            MESSAGES.ERRORS.UNSUPPORTED_COMMISSION_MODEL
 
         if self.initialized:
             raise Exception(MESSAGES.ERRORS.OVERRIDE_COMMISSION_POST_INIT)