Fix regex deprecation warning

[terriko]

pytest tossed the following warning at me:

=============================== warnings summary ===============================
cve_bin_tool/cvedb.py:153
  /home/terri/Code/cve-bin-tool/cve_bin_tool/cvedb.py:153: DeprecationWarning: invalid escape sequence \/
    META_REGEX = re.compile("https:\/\/.*\/json\/.*-[0-9]*\.[0-9]*-[0-9]*\.meta")

-- Docs: https://docs.pytest.org/en/latest/warnings.html

We should probably fix that. I believe you just need to correct the string to add an r at the front so it's treated as a "raw" string.

So on line 153 above, change it to be r"https instaed of "https