fix/jwtSync

fixes spotify#68

per example in related issue; time sync issue can occur when setting issued at to be the current time. This sets the issued at to be 1 minute earlier to avoid this.

Tested locally for 2 days with no issues.

src/main/java/com/spotify/github/v3/clients/JwtTokenIssuer.java

@@ -36,6 +36,7 @@ public class JwtTokenIssuer {
 
   private static final SignatureAlgorithm SIGNATURE_ALGORITHM = SignatureAlgorithm.RS256;
   private static final long TOKEN_TTL = 600000;
+  private static final long TOKEN_ISSUED = 6000;
 
   private final PrivateKey signingKey;
 
@@ -74,7 +75,7 @@ public String getToken(final Integer appId) {
         .setIssuer(String.valueOf(appId))
         .signWith(signingKey, SIGNATURE_ALGORITHM)
         .setExpiration(new Date(System.currentTimeMillis() + TOKEN_TTL))
-        .setIssuedAt(new Date())
+        .setIssuedAt(new Date(System.currentTimeMillis() - TOKEN_ISSUED))
         .compact();
   }
 }