Improve browser support for dimension utils used with flexbox.

When dimension utils are used on flex items, we need to ensure that their `flex-*` properties (grow, shrink, basis) are reset in order for the width specified by the util to take effect. Previously this was set with `flex: none` but this has issues in some older flexbox implementations. Using `flex: initial` solves the issue.

Status: **Ready to merge**

Reviewers: **@kpeatt, @jeffkamo **
Ticket: **fixes #50** 
Linked PRs: **N/A**

## Changes
- Change flexbox property reset from `flex: none` to `flex: initial`.