Typo fix in package.json (change ^ to >)

The package.json file for html-snapshots is causing problems when I try to install other npm packages that depend on phantomjs. Looking at the package.json file in this repo, I see the line

```
"dependencies": {
  "phantomjs": "^1.7.1",
...
```

The **^** character doesn't mean anything in a package.json file.

If the package depends on a phantomjs version greater than 1.7.1, what we need is

```
"dependencies": {
  "phantomjs": ">1.7.1",
...
```

Per version range notation in https://www.npmjs.org/doc/json.html#dependencies