[doc, irtmodel] documentation string should be compiled by latex

[YoheiKakiuchi]

This PR fix the error,

[209] (./irtmodel-func.tex [210]
! Missing $ inserted.
<inserted text> 
                $
l.22 ...of this joint in SI(m/s^2, rad/s^2) unit.}
                                                  
? 

[snozawa]

LGTM.

[k-okada]

If human find some error before travis finds, we need to update Travis, specially error simple enough to catch like this. So start working -> #451

[k-okada]

https://travis-ci.org/euslisp/jskeus/jobs/265007029, line 2460 - 2465, How can I reproduce this error ?

Overfull \hbox (60.43997pt too wide) in paragraph at lines 1903--1909
\JY1/mc/m/it/10 ることができる ． この座標がどこにあるか確認するためには \OT1/c
mtt/m/n/10 (send (car (send *kettle* :handle)) :draw-on :flush t)
[209] (./irtmodel-func.tex [210]
Underfull \vbox (badness 10000) has occurred while \output is active [211]

[snozawa]

Thanks you for starting #451.

If human find some error before travis finds, we need to update Travis, specially error simple enough to catch like this. So start working -> #451

I completely agree with this point.

[k-okada]

Finally add travis check on #453