dashboard: increase notification log to 200 lines

dashboard: increase notification log to 200 lines

The go command lists all packages that were built.
A recent breakage notification included this text:

$ tail -100 < log
ok      errors
ok      exp/ebnf
?       exp/ebnflint [no test files]
ok      exp/gotype
ok      exp/norm
ok      exp/spdy

If the breakage occurred before this point it would
not be visible in the output.

R=golang-dev, gri, kevlar
CC=golang-dev
https://golang.org/cl/5504109