Port matcher matches port by a substring but not the whole number

The match expression for grep is not terminated now and that leads to errors when matching substrings.

For example:
```
describe port(77) do
  it { should be_listening }
end
```
This case would match ports 77, 777 and 777

The pull request should fix the issue.
I believe it should be compatible to all systems but It was tested only on Ubuntu 12.04