Fix check for antiword binary

[tommygnr]

We are checking for the existence of the string ‘antiword: No such file
or directory’ not its absence so the test should be !== -1
If checking for === -1 we will also fail here when the binary exists
but the file is not a word doc (as is the case in the testForBinary
function)

Also add a check for antiword: not found as that is the error message
I receive on my platform (Ubuntu 16.04)

[dbashford]

Looks like this is off in a couple places, will probably simplify to not found as that is what I'm getting locally myself.