This fixes a regression with `check_file_contain_within`

With [this PR](https://github.com/serverspec/specinfra/pull/191) a
regression was introduced with the `before` and `after` matchers when
they aren't being used together.

With PR 191 the following:

```
  it { should contain('rspec').before(/^end/) }
```

would have resulted in:

```
  sed -n 1,/\\^end/p Gemfile | sed -n 1,/\\^end/p | grep -qs -- rspec - || sed -n 1,/\\^end/p Gemfile | sed -n 1,/\\^end/p | grep -qFs -- rspec -
```

as well as:

```
    it { should contain('rspec').after(/^group :test do/) }
```

would result in:

```
    sed -n /\\^group\\ :test\\ do/,\\$p Gemfile | sed -n 1,\\$p | grep -qs -- rspec - || sed -n /\\^group\\ :test\\ do/,\\$p Gemfile | sed -n 1,\\$p | grep -qFs -- rspec -
```

Notice in both scenarios the `sed` command is unnecessarily duplicated.

While neither of these should actually cause an assertion to pass when
it should fail, they also do not add anything for either situation.
Merge pull request #200 from mattjmorrison/before_and_after_regression

This fixes a regression with `check_file_contain_within`