Fix Phone Number Regex

[kimshitong]

Closes #196

Make it become more restrictive that phone numbers should start with 6, 8, or 9 and be 3 or 8 digits long. eg: 911, 81754211

[codecov[bot]]

Codecov Report

Merging #239 (1634c03) into master (8fd3fc2) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #239   +/-   ##
=========================================
  Coverage     81.04%   81.04%           
  Complexity      938      938           
=========================================
  Files           137      137           
  Lines          2823     2823           
  Branches        275      275           
=========================================
  Hits           2288     2288           
  Misses          502      502           
  Partials         33       33           

Files	Coverage Δ
...rc/main/java/seedu/address/model/person/Phone.java	93.33% <ø> (ø)

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[yanghengtang]

This is not allowed for v1.4

[derekjxtan]

This shall be planned enhancements in DG