[BUG] mattermost self-signed certificate

[altairo2087]

Describe the bug
I have private mattermost server with self-signed certificate. I'm tried to add custom CA to docker container.

INFO[2019-04-16T06:44:07Z] Allowed Events - map[{Resource:clusterrole Namespace:all}:true {Resource:configmap Namespace:all}:true {Resource:daemonset Namespace:all}:true {Resource:deployment Namespace:all}:true {Resource:ingress Namespace:all}:true {Resource:job Namespace:all}:true {Resource:namespace Namespace:all}:true {Resource:node Namespace:all}:true {Resource:persistentvolume Namespace:all}:true {Resource:persistentvolumeclaim Namespace:all}:true {Resource:pod Namespace:all}:true {Resource:role Namespace:all}:true {Resource:rolebinding Namespace:all}:true {Resource:secret Namespace:all}:true {Resource:service Namespace:all}:true]
DEBU[2019-04-16T06:44:07Z] Registering the filter&{}
DEBU[2019-04-16T06:44:07Z] Registering the filter&{}
DEBU[2019-04-16T06:44:07Z] Registering the filter&{}
INFO[2019-04-16T06:44:07Z] Starting controller
INFO[2019-04-16T06:44:07Z] Starting mattermost bot
panic: runtime error: invalid memory address or nil pointer dereference
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x115de22]

goroutine 53 [running]:
github.com/infracloudio/botkube/vendor/github.com/mattermost/mattermost-server/model.(*WebSocketClient).Listen.func1.1(0x0)
        /go/src/github.com/infracloudio/botkube/vendor/github.com/mattermost/mattermost-server/model/websocket_client.go:101 +0x22
panic(0x1314c20, 0x2322130)
        /usr/local/go/src/runtime/panic.go:522 +0x1b5
github.com/infracloudio/botkube/vendor/github.com/mattermost/mattermost-server/model.(*WebSocketClient).Listen.func1(0x0)
        /go/src/github.com/infracloudio/botkube/vendor/github.com/mattermost/mattermost-server/model/websocket_client.go:109 +0x64
created by github.com/infracloudio/botkube/vendor/github.com/mattermost/mattermost-server/model.(*WebSocketClient).Listen
        /go/src/github.com/infracloudio/botkube/vendor/github.com/mattermost/mattermost-server/model/websocket_client.go:99 +0x3f

To Reproduce
Steps to reproduce the behavior:

1. Add custom CA certificate to /usr/local/share/ca-certificates
2. Run docker container with /bin/sh -c "apk add --update --no-cache ca-certificates && update-ca-certificates --fresh && /go/bin/botkube"

Additional context
Run with defaults:

/bin/sh -c "/go/bin/botkube"

INFO[2019-04-16T06:57:01Z] Allowed Events - map[{Resource:clusterrole Namespace:all}:true {Resource:configmap Namespace:all}:true {Resource:daemonset Namespace:all}:true {Resource:deployment Namespace:all}:true {Resource:ingress Namespace:all}:true {Resource:job Namespace:all}:true {Resource:namespace Namespace:all}:true {Resource:node Namespace:all}:true {Resource:persistentvolume Namespace:all}:true {Resource:persistentvolumeclaim Namespace:all}:true {Resource:pod Namespace:all}:true {Resource:role Namespace:all}:true {Resource:rolebinding Namespace:all}:true {Resource:secret Namespace:all}:true {Resource:service Namespace:all}:true]
DEBU[2019-04-16T06:57:01Z] Registering the filter&{}
DEBU[2019-04-16T06:57:01Z] Registering the filter&{}
DEBU[2019-04-16T06:57:01Z] Registering the filter&{}
INFO[2019-04-16T06:57:01Z] Starting controller
INFO[2019-04-16T06:57:01Z] Starting mattermost bot
ERRO[2019-04-16T06:57:06Z] There was a problem pinging the Mattermost server URL: https://private.server
Error: https://private.server/api/v4/config/client?format=old: model.client.connecting.app_error, Get https://private.server/api/v4/config/client?format=old: x509: certificate signed by unknown authority
ERRO[2019-04-16T06:57:06Z] Error in connecting to Mattermost team TEAM
Error: https://private.server/api/v4/teams/name/TEAM: model.client.connecting.app_error, Get https://private.server/api/v4/teams/name/TEAM: x509: certificate signed by unknown authority

[PrasadG193]

Fixed with #83