Fix or acknowledge R >=3.5.0 dependency

[avahoffman]

Installing from console yields

─  checking for LF line-endings in source and make files and shell scripts
─  checking for empty or unneeded directories
     NB: this package now depends on R (>= 3.5.0)
     WARNING: Added dependency on R >= 3.5.0 because serialized objects in
     serialize/load version 3 cannot be read in older versions of R.
     File(s) containing such objects:
       ‘dasehr/data/CO_heat_ER.RData’ ‘dasehr/data/CO_heat_ER_byage.RData’
       ‘dasehr/data/CO_heat_ER_bygender.RData’
       ‘dasehr/data/CO_heat_ER_long.RData’
       ‘dasehr/data/CO_heat_ER_wide.RData’
       ‘dasehr/data/calenviroscreen.RData’
       ‘dasehr/data/covid_wastewater.RData’ ‘dasehr/data/er_CO_county.RData’
       ‘dasehr/data/er_CO_statewide.RData’ ‘dasehr/data/haa5.RData’
       ‘dasehr/data/nitrate.RData’ ‘dasehr/data/yearly_co2_emissions.RData’
─  building ‘dasehr_0.1.0.tar.gz’

This warning might scare folks.