Download functions in /data-raw have old links

[guslipkin]

The links used in airlines, flights, and planes are no longer valid

Can be fixed immediately:

• planes: can be replaced with https://registry.faa.gov/database/ReleasableAircraft.zip
• airlines: can be replaced with https://www.transtats.bts.gov/Download_Lookup.asp?Y11x72=Y_haVdhR_PNeeVRef

I'm working on fixing:

• flights: bts.gov no longer uses the same url format. Instead of year and month being specified in the URL, there is now an input form and all parameters are past through that