travis: optimize lint

[AurelienGasser]

1. Lint only our modules:

• Running golint doesn't lint the errors/ package.
• Running golint ./... lints the vendors/ package, which is wrong too.
• => Use go list with a grep in order to lint only the packages we wrote

2. Try building the chaincode even if linting fails

Rationale: I assume it's always interesting to see if the package can build, even if linting failed.

[AurelienGasser]

Running golint after go build leads to the error

chaincode:1:1: illegal character U+007F

I'll try a new approach: lint before the build, but try building even if linting fails.

[AurelienGasser]

Looks like it works:

• It caught the error in the main directory
• It caught the error in the errors/ subdirectory
• It still tried to go build the package
• The CI build gets marked as "failed" out because of the lint errors

I'll remove the intentional lint errors.

[AurelienGasser]

This is ready for review.

Sorry for the noise !

[AurelienGasser]

Today I learned: adding : (colon followed by a space) to a travis command makes the YAML invalid! Because : is part of the YAML syntax.