Fix arwu subject

Includes updates to fix ARWU subject scrape bug (#12 (closed)) and updates to data sets and other minor cleaning.

Details of updates:

• ur_scrape_arwu_subject fixed to handle scraping of 2020 (fixed #12 (closed)). Due to change in format of data on ARWU website for 2020 subject ranks, error caused when trying to attach column names (as now additional National/Regional Rank column). Added function get_subject_colnames that checks which year data is requested for and returns appropriate column names. As part of the, also returns new Q1 metric column name that has replaced PUB metric
• Updated data sets for Times and QS to include 2020 data
• Fixes bug that was caused in Times example documentation due to outdated JSON URLs for rankings. Replaced to include correct JSON URLS
• Minor updates to documentation as relevant
• Replace as_data_frame with as_tibble (as_data_frame is now deprecated)