Update Arduino IDE version to 1.8.7

[frazar]

This kind of changes could be performed automatically by setting the CI script to always use the latest version of the Arduino.

[frazar]

The build seems to be hitting this unresolved bug in the Arduino IDE.

In particular, since commit e209f7d, Travis builds of arduino-lmic use the cached .arduino15 folder from previous builds. Thus, when the command

arduino --install-boards mcci:samd

is run, it finds that the mcci:samd board is already installed, prints the following message

java.lang.Exception: Platform is already installed!
	at cc.arduino.contributions.packages.ContributionInstaller.install(ContributionInstaller.java:77)
	at processing.app.Base.<init>(Base.java:324)
	at processing.app.Base.main(Base.java:145)

and fails with error code 255.

[terrillmoore]

Thanks for the request.

I'm perhaps too conservative about using the "latest" version of the IDE in CI testing.

Anyway, I think the thing to do is to check what boards are installed and then don't try to install things that are already installed. That's easy enough to do in the ci script. I think I need to resolve this, will first fix the caching issue, then accept this pull request, then look into #140.

[terrillmoore]

Adjusted to take into account fixes from #143.