starts using rust 1.15 (remove redundant linkage that cause compilation

fail)
13 jobs from develop in 34 minutes and 46 seconds
Status Job ID Name Coverage
  Test
passed #2772
test:cargo:api

02:02

passed #2774
test:cargo:compiler

05:01

passed #2773
test:cargo:ir

02:25

passed #2775
test:cargo:runtime

02:06

passed #2777
testjit:binops

03:44

passed #2778
testjit:cmpops

02:31

passed #2779
testjit:controlflow

02:11

passed #2780
testjit:convops

02:08

passed #2781
testjit:double

02:37

passed #2782
testjit:memops

02:12

passed #2776
testjit:milestones

02:13

passed #2783
testjit:otherops

02:09

failed #2784
testjit:rpython

03:21