Update bioguide scraper to handle 'ca.' in years

Charles Slade's birth-death range is listed as `ca. 1797-1834`,
which broke the scraper. The scraper only expected
`c?\.?`, which I have changed to `(c(a\.)?)?`. Tested with a
full scrape without issues.

BTW, I also have an updated scrape (includes 113th congressmen) which I could send you to update bioguide[123].csv on govtrackdata. Interested?