update AWS SDK to v1.15.90

Picking up support for credential_source. This allows assuming a role
while using profile creds from ec2 instance metadata:
aws/aws-sdk-go#2201.

go.mod

@@ -1,8 +1,10 @@
 module github.com/segmentio/chamber
 
 require (
-	github.com/aws/aws-sdk-go v1.15.46
+	github.com/aws/aws-sdk-go v1.15.90
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/go-ini/ini v1.25.4 // indirect
+	github.com/inconshreveable/mousetrap v1.0.0 // indirect
 	github.com/magiconair/properties v1.8.0
 	github.com/pkg/errors v0.8.0
 	github.com/pmezard/go-difflib v1.0.0 // indirect

go.sum

@@ -1,9 +1,11 @@
-github.com/aws/aws-sdk-go v1.15.46 h1:gX48KxOq3XtZKDdtkBFTh/emUuxVUX22sbfO2wa/7d0=
-github.com/aws/aws-sdk-go v1.15.46/go.mod h1:mFuSZ37Z9YOHbQEwBWztmVzqXrEkub65tZoCYDt7FT0=
+github.com/aws/aws-sdk-go v1.15.90 h1:W8qBrxaKIK6nPqO/UUDcjgokmS9wJQ45D273clpoBA4=
+github.com/aws/aws-sdk-go v1.15.90/go.mod h1:es1KtYUFs7le0xQ3rOihkuoVD90z7D0fR2Qm4S00/gU=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/go-ini/ini v1.25.4 h1:Mujh4R/dH6YL8bxuISne3xX2+qcQ9p0IxKAP6ExWoUo=
 github.com/go-ini/ini v1.25.4/go.mod h1:ByCAeIL28uOIIG0E3PJtZPDL8WnHpFKFOtgjp+3Ies8=
+github.com/inconshreveable/mousetrap v1.0.0 h1:Z8tu5sraLXCXIcARxBp/8cbvlwVa7Z1NHg9XEKhtSvM=
+github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
 github.com/jmespath/go-jmespath v0.0.0-20160202185014-0b12d6b521d8 h1:12VvqtR6Aowv3l/EQUlocDHW2Cp4G9WJVH7uyH8QFJE=
 github.com/jmespath/go-jmespath v0.0.0-20160202185014-0b12d6b521d8/go.mod h1:Nht3zPeWKUH0NzdCt2Blrr5ys8VGpn0CEB0cQHVjt7k=
 github.com/magiconair/properties v1.8.0 h1:LLgXmsheXeRoUOBOjtwPQCWIYqM/LU1ayDtDePerRcY=