Don't choke on comments or blank lines in requirements.in

Without this, `pundler install` will choke if `requirements.in` has comments or blank lines.

Here's a sample `requirements.in` that I'm trying to work with:

```
# requirements.in

--index-url http://packages.mycompany.com/index

# ---------------------------------------------------------------------------
# SurveyMonkey packages
# ---------------------------------------------------------------------------

rattr>=0.6
smlib.appstatus>=0.0.13
smlib.log>=0.0.7

# @todo: Use smsdk.surveysvc instead of smlib.service
# smsdk.surveysvc
smlib.service>=0.1.33
smsdk.usersvc>=0.0.8

smlib.smsqlalchemy>=0.13
smlib.traceback>=0.3
smlib.validate>=0.0.10

# ---------------------------------------------------------------------------
# External packages
# ---------------------------------------------------------------------------

cornice>=0.16.2
pyramid>=1.4.5
pyramid_jinja2>=1.8
SQLAlchemy>=0.7.6
pymssql>=2.1.0
redis
waitress>=0.8.5
gunicorn
vcrpy
six>=1.6.1

# Python 2 only
Paste>=1.7.5.1
PasteScript>=1.7.5
gevent>=1.0
smlib.cache[memcached]==1.0.6-fix-invalidate-method-before-calling-method

# Python 3 only
# smlib.cache[redis]>=1.0.1, <2.0.0
```