test: Remove travis checkout depth
test: Remove travis checkout depth

Tests on branches of non-head commits are failing, because the depth of
1 doesn't allow checking them out.

Remove `depth` as was the case before fa44af5cd2152a21da9ef3e48c073a668bf2df27,
so that Travis can determine the minimum depth to check out.

peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin