Fixed bug in populate function

/var/node/node_modules/mlcl_elastic/node_modules/mongolastic/index.js:80
    if(options.ref) {
              ^
TypeError: Cannot read property 'ref' of undefined
    at populateReferences (/var/node/node_modules/mlcl_elastic/node_modules/mongolastic/index.js:80:15)
    at /var/node/node_modules/mlcl_elastic/node_modules/mongolastic/index.js:109:13
    at /var/node/node_modules/async/lib/async.js:125:13
    at Array.forEach (native)
    at _each (/var/node/node_modules/async/lib/async.js:46:24)
    at Object.async.each (/var/node/node_modules/async/lib/async.js:124:9)
    at /var/node/node_modules/mlcl_elastic/node_modules/mongolastic/index.js:106:17
    at /var/node/node_modules/async/lib/async.js:125:13
    at Array.forEach (native)
    at _each (/var/node/node_modules/async/lib/async.js:46:24)

Signed-off-by: Alexander Knapstein <alexander.knapstein@inspirationlabs.com>