Fixed first character removal

[bezineb5]

The library was crashing when the sentence contained multiple $ characters, such as:
$GPTXT,01,01,02,u-blo?$GPTXT,01,01,02,u-b$GPTXT,01,01,02,u-bl?$GPTXT,01,01,02,u-blox ag - www.u-blox.co?$GPTXT,01,01,02,u-blox ag - www.u-blox.com*50
(I agree it's invalid, but that makes the library crash)

[coveralls]

Coverage remained the same at 86.588% when pulling 57db201 on bezineb5:fix-trim-$ into 523439f on adrianmo:master.

[adrianmo]

Hi @bezineb5,

I'm not sure what is the problem you are trying to fix. The example you provided doesn't look good and the sentence type (ie. GPTXT) is not even supported by this lib yet. Can you please clarify your use case?

That said, I think you brought a good point (maybe unintentionally?). If there is more than one $ in the sentence, we should probably only consider the last one and ignore the ones in the left.

Check out this example: https://play.golang.org/p/1beUJTWMvO

I've added a few sentences and tested the current solution, your proposed solution, and a new one that might work in the scenario I mentioned.

Let me know what you think.

[bezineb5]

I'm reworking it with a difference pull request to take your comments into account.