FIX: don't create 32 bit int type for int64 column (GH7433)
Closes #7433

Replace the sqlalchemy `Integer` type with `BigInteger` as the default integer type in pandas is int64. 
Probably we could be more specific in the type conversion (check for the exact integer type).

Tests are passing. @mangecoeur @danielballan OK with this?
 Looks good.
 Could someone test it with oracle?
 I see that there are on travis a lot of warnings after merging this. So the tests pass, but raise warnings like: `/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/sqlalchemy/engine/reflection.py:46: SAWarning: Did not recognize type 'BIGINT' of column 'index'`

I didn't see this warning, and after testing, I don't see it with sqlalchemy 0.9.6 and 0.8.3, but I do see it with 0.8.1 (the version used on travis for build 3).

As the tests pass, I assume this is not a problem. But anybody an idea to prevent this warning is raised when users have an older version of sqlalchemy?
 so u want to put in place one/both of those newer versions on Travis? which ones should I replace?
 Well, already different version are used on different builds on travis. The older version 0.7.1 is used on py2.6, 0.8.1 is used on py2.7 and 0.9.1 is used on py3.3. So the older version of 0.7.1 should be kept to test compatibility with older sqlalchemy, the other ones could be updated to the latest sqlalchemy version. 
 This issue was solved in 0.8.2 (https://bitbucket.org/zzzeek/sqlalchemy/issue/2764/make-sqlite-handle-bigint).

So, or we live with the fact that users with older versions of sqlalchemy get this warning, or we can also try to 'fix' it ourselves with something like: https://review.openstack.org/#/c/23883/4/nova/db/sqlalchemy/__init__.py (from, a [similar nova bug](https://bugs.launchpad.net/nova/+bug/1147879))
 I wouldn't worry about in that case; changing the 3.4 build to use 0.9.6
 You can also change the 2.7 build to use 0.9.6, then the warning don't noise the travis test output (and the older 0.7.1 is still tested in 2.6)
 both done (Travis still building master though)
